Who Guards Our Doors?

Can you explain the significance of having a mezuzah on your house and its power to guard the household, as opposed to it being just a superstitious belief? Thank you!

AskTheRabbi.org answered:

The mitzvah to put up a mezuzah is taught twice in the Torah. The first two paragraphs of the “Shema Yisrael” are written in the mezuzah since these two paragraphs mention mezuzah.
Jewish tradition teaches that in the merit of the mezuzah God protects our houses. Why should this be? There is no magic involved, and the explanation is relatively simple. Because the degree of God’s involvement in our lives is directly proportional to the degree that we are conscious and aware of Him, the more that we are reminded of God’s presence, the more of His presence we will experience. The mezuzah is a vital element in creating and maintaining that awareness, and Divine protection is the result.
This idea of the mezuzah and “home security” is related by the Talmud in a fascinating episode:
Onkelos, the brilliant nephew of the Roman Emperor Titus, converted to Judaism and became a disciple of the Jewish Sages. Hearing this, Titus sent a brigade of soldiers to bring him back to Rome. But when Onkelos engaged the soldiers in discussion and showed them the beauty of Torah, they converted to Judaism.
Titus then sent another brigade, instructing them not to speak to Onkelos, but with the same result: They too converted to Judaism.
Finally Titus sent a third brigade and instructed them not even to listen to Onkelos. When they were leading Onkelos away, Onkelos placed his hand on the mezuzah and said, “What is that?”
“You tell us,” the soldiers said.
“Normally, a human king sits inside and his servants stand outside and guard him. But, the Holy One, Blessed is He, His servants are inside and He guards them from outside.”
They too converted. Titus sent no more soldiers.